DE10326741B4 - A process for producing a cellulose-based microporous membrane and a cellulose-based microporous membrane obtained by the process - Google Patents

Abstract

A process for producing a cellulose-based microporous membrane having a porous protective layer as a first membrane surface and a porous protective layer as a second membrane surface and a porous sponge-like structure between these two protective layers, the sponge-like structure consisting of two isotropic regions, and the pores of the first isotropic region, which is adjacent to the first membrane surface, smaller than the pores in the protective layer, which is the first membrane surface, but larger than the pores of the second isotropic region, and the pores of the second isotropic region are smaller than the pores in the protective layer, which second membrane surface, the method comprising the steps of: (a) preparing a homogeneous casting solution comprising a cellulosic material comprising at least one volatile solvent for the cellulosic material and at least one volatile non-solvent (b) pouring the casting solution on a pad to a thin film under inert gas at a predetermined temperature, (c) evaporating the volatile solvent and the volatile non-solvent under a controlled atmosphere consisting of the at least one volatile solvent comprising at least one volatile non-solvent and the inert gas until a white surface forms on top of the microporous membrane forming the casting solution film, and (d) drying the formed microporous membrane by overflowing the membrane surface the inert gas.

Description

The present invention relates to an environmentally friendly process for producing a cellulose-based microporous membrane and to a cellulose-based microporous membrane obtained by the process according to the claims, respectively. The membrane of the invention is characterized by the specific structure in particular by a high filtration performance and a high Filtrationsstandzeit.

In the prior art are symmetrical membranes based on cellulose (see. DE 44 38 381 A1 ) and asymmetric membranes based on synthetic polymers, such as polysulfone polymers known (see. EP 1 118 377 A2 ).

Membranes with asymmetric structure are characterized by a pore size gradient across the membrane thickness. Usually asymmetric membranes have the pores with the smallest diameters on the side used for the preparation used for the preparation ("air or gas side") and the pores with the largest diameters on the pad-facing side of the membrane. For asymmetric membranes, the pore size increase across the membrane thickness is generally gradual. Symmetrical membranes have a substantially constant pore diameter over the entire thickness of the membrane. A region of pores of constant size is called an isotropic region.

US 2003/0038081 A1 describes cellulose membranes of asymmetric structure having a first porous layer and a second porous layer and a porous support structure between the first and second porous layer. Furthermore, in US 2003/0038081 A1 describes a membrane having a so-called "funnel-with-a-neck" structure with both an asymmetric and an isotropic region, the pores in this isotropic region having a constant pore size.

EP 0 723 993 B1 describes a cellulose acetate solution comprising an ester having 3 to 12 carbon atoms as a solvent, and a process for producing a cellulose acetate film using this cellulose acetate solution.

DE 199 25 929 C2 describes a microfiltration membrane based on cellulose acetate containing hydrophilic synthetic silicic acid and a conventional method for producing this silicic acid-containing membrane.

US 5 522 991 A discloses an ultrafiltration membrane comprising a microporous base-resistant carrier and an ultrafiltration layer of a cellulose ester or cellulose.

DE 101 50 073 A1 describes a multilayer filter medium consisting of a layer of filter active material covered on at least one side with a laminating material.

US 4,894,157 A describes a continuous process for the production of microporous cellulose membranes.

DE 197 52 143 A1 discloses a filter element in which multiple layers of a filter medium are joined together. On the inflow side, at least one layer of a filter medium with a low separation efficiency and downstream of at least one layer of a filter medium with a high separation efficiency is arranged.

DE 695 22 209 T2 describes a method for surface modification of a polymeric membrane and the resulting membrane.

US 3,884,801 A describes a method of making a porous reverse osmosis membrane having a gradual pore size gradient across the membrane thickness.

It is an object of the present invention to provide a cellulose-based membrane which is said to have excellent filtration performance and durability, and to provide a particularly environmentally friendly process for producing such a membrane.

These objects are achieved by the embodiments of the present invention characterized in the claims.

More specifically, the present invention provides a cellulose-based microporous membrane obtained by the method of the present invention having a porous (integral) protective layer as a first membrane surface and a (integral) porous protective layer as a second membrane surface and a porous sponge-like structure between these two protective layers, the sponge-like structure is composed of two isotropic regions, and the pores of the first isotropic region adjacent to the first membrane surface are smaller than the pores in the protective layer which is the first membrane surface but larger than the pores of the second isotropic region, and the pores of the first isotropic region second isotropic region are smaller than the pores in the protective layer, which represents the second membrane surface.

By "(integral) porous protective layers" is meant in the context of the present invention that the porous protective layers are an integral part of the membrane structure. This means that the (integral) porous protective layers in the course of the process according to the invention form on the upper side of the microporous membrane or on the side facing the substrate, without having to be applied separately. In the same way, the specific isotropic regions are formed in the course of the process according to the invention.

The relative terms "smaller pores" and "larger pores" in the context of the present invention are to be understood as meaning that the pores have a smaller or larger average pore diameter.

The microporous membrane of the present invention has such a structure between the top of the membrane (adjacent to the first isotropic region) and the bottom of the membrane (adjacent to the second isotropic region) that the pores of the first membrane surface are generally larger than the pores of the second membrane surface , The microporous membrane according to the invention is particularly well suited, for example, for the filtration of fluids in the microfiltration range.

The pore density, ie the number of pores per unit area (the pore density determination is made visually on scanning electron micrographs) in the porous protective layers is preferably in the range of 1 × 10 ⁴ to 1 × 10 ⁶ pores / mm ² . The pore density in the porous protective layer, which is the first membrane surface (gas side), is preferably in a range of 1 × 10 ⁴ to 5 × 10 ⁵ pores / mm ² . The pore density in the porous protective layer constituting the second membrane surface (side of the base) is preferably in a range of 3 × 10 ⁵ to 1 × 10 ⁶ pores / mm ² . More preferably, the pore density in the porous protective layer constituting the first membrane surface is in a range of 3 × 10 ⁴ to 3 × 10 ⁵ pores / mm ^2, and the pore density in the porous protective layer constituting the second membrane surface is in a range from 4 × 10 ⁵ to 7.5 × 10 ⁵ pores / mm ² .

The pores in the porous protective layer, which is the first membrane surface, preferably have a mean diameter in a range of 0.4 .mu.m to 15 .mu.m, particularly preferably from 0.8 .mu.m to 5.0 .mu.m. The pores in the porous protective layer, which is the second membrane surface, preferably have a mean diameter in a range of 0.1 .mu.m to 3.0 .mu.m, particularly preferably 0.5 .mu.m to 2.0 .mu.m.

The sponge-like structure of the microporous membrane of the present invention consists of two discrete but integrally arranged isotropic regions each having a substantially uniform pore size which changes abruptly at the boundary between the first isotropic region and the second isotropic region. As used herein, the term "isotropic region" generally refers to a region within the microporous membrane that has a substantially uniform (average) pore size. The pores in the (two) isotropic regions preferably differ by a factor of 1.4 to 16, more preferably by a factor of from 2 to 12, and most preferably by a factor of from 2.5 to, based on the mean pore diameter 6. The average pore diameter in the first isotropic region ranges from about 0.05 μm to about 0.30 μm (gas side). The mean pore diameter in the second isotropic region is in a range of about 0.20 μm to 0.80 μm (backing side).

In the microporous membrane of the present invention, one of the isotropic regions preferably constitutes 20% to 80%, more preferably 40% to 60%, and most preferably about 50% of the thickness of the porous sponge-like structure.

The microporous membrane of the present invention can be prepared in a total thickness (after drying) in a range of about 5 μm to about 1000 μm, with a total thickness of 20 μm to 500 μm μm is preferred and a total thickness of 60 μm to 180 μm is particularly preferred. The porous protective layers of the microporous membrane according to the invention have a monolayer thickness of <1 .mu.m.

The first and the second isotropic region of the microporous membrane of the present invention has a thickness in a range of 15 μm to 200 μm, with a thickness of 20 μm to 160 μm being preferred, and a thickness in a range of 30 μm to 90 μm particularly is preferred. The first and second isotropic regions together have a thickness in a range of 80 μm to 250 μm, with a thickness of 90 μm to 190 μm being preferred.

The cellulose-based microporous membrane of the present invention is a microporous membrane composed essentially of a cellulose-based material. Preferably, the cellulose-based microporous membrane according to the invention is composed exclusively of a cellulose-based material and optionally conventional additives. Preferred cellulosic materials are cellulose esters such as cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetobutyrate, cellulose acetopropionate, cellulose nitrate, etc., or mixtures thereof. Regenerated cellulose made from a post-saponification of cellulose acetates is also a preferred material for the microporous membrane of the present invention. Preferred cellulose esters are cellulose acetates, with cellulose triacetate, cellulose diacetate and mixtures of cellulose triacetate and cellulose diacetate being particularly preferred.

The weight-average molecular weight (Mw) of the cellulose-based material such as cellulose acetate of cellulose diacetate and cellulose triacetate is usually in a range of 1 × 10 ⁵ to 5 × 10 ⁵ g / mole.

Preferably, the cellulose-based material is a mixture of cellulose triacetate having an acetic acid content in a range of about 58% to 62.5% and cellulose diacetate having an acetylation degree in a range of 51% to about 57%, wherein a weight ratio of cellulose triacetate: Cellulose diacetate of 1.5: 1 to 0.8: 1 is particularly preferred.

To stabilize the microporous membrane according to the invention, for example to increase the mechanical strength, a reinforcing material, such as a nonwoven material, may be integrated into the membrane. The reinforcing material may, for example, have a mesh structure, a lattice structure or a woven structure of conventional materials. Suitable reinforcing materials are any materials that do not form diffusion channels in conjunction with the membrane polymer. Preferably, a nonwoven fabric made of polymers is used as reinforcement material for the membrane.

• (a) Herstellen einer homogenen Gießlösung, die ein Material auf Cellulosebasis umfaßt, die mindestens ein leichtflüchtiges Lösungsmittel für das Material auf Cellulosebasis und mindestens ein flüchtiges Nicht-Lösungsmittel für das Material auf Cellulosebasis enthält,
• (b) Gießen der Gießlösung auf eine Unterlage zu einem dünnen Film unter einem inerten Gas bei einer vorbestimmten Temperatur, gegebenenfalls Auftragen eines Verstärkungsmaterials, wie eines Netzes oder Gewebes,
• (c) Abdampfen des leichtflüchtigen Lösungsmittels und des flüchtigen Nicht-Lösungsmittels unter einer kontrolliert zusammengesetzten Atmosphäre, die aus dem mindestens einen leichtflüchtigen Lösungsmittel, dem mindestens einen flüchtigen Nicht-Lösungsmittel und dem inerten Gas besteht, bis zur Ausbildung einer weißen Oberfläche auf der Oberseite der sich aus dem Gießlösungsfilm bildenden mikroporösen Membran und
• (d) Trocknen der gebildeten mikroporösen Membran durch Überströmen der Membranoberfläche mit dem inerten Gas.

Further, the present invention provides a process for producing the above-described microporous membrane comprising the steps of:

• (a) preparing a homogeneous casting solution comprising a cellulosic material containing at least one volatile solvent for the cellulosic material and at least one volatile non-solvent for the cellulosic material,
• (b) pouring the casting solution onto a base to form a thin film under an inert gas at a predetermined temperature, optionally applying a reinforcing material, such as a mesh or fabric,
• (c) evaporating the volatile solvent and the volatile non-solvent under a controlled composition atmosphere consisting of the at least one volatile solvent, the at least one volatile non-solvent and the inert gas, to form a white surface on top of the from the casting solution film forming microporous membrane and
• (D) drying the microporous membrane formed by overflowing the membrane surface with the inert gas.

An inert gas is understood as meaning a gas or a gas mixture which contains no substances which react with the components of the casting solution or lead to the precipitation of a polymer from the casting solution. Such substances would be, for example, water or alcohols.

By the method according to the invention, in contrast to conventional, for example in US 2003/0038081 A1 asymmetric membranes described, obtained a microporous membrane whose pores with the largest diameters are located on the opposite side of the pad and their pores with the smallest diameters on the pad facing side.

With regard to the microporous membrane which can be produced by the process according to the invention, reference is made to the above definitions.

The homogeneous casting solution described in step (a) may contain, in addition to the cellulosic material, the at least one volatile solvent for the cellulosic material and the at least one volatile non-solvent for the cellulosic material, conventional additives for membrane casting solutions, for example glycerol and / or or polyethylene glycol, or plasticizers such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate or dioctyl phthalate, etc. The homogeneous casting solution can be prepared by a conventional method, for example by dispersing a mixture of a cellulose-based material and at least one volatile solvent for a time sufficient to form a gel particle-free, cloudy solution, generally 30 minutes to 5 hours, by subsequent low temperature solvation, as well in EP 0 723 993 B1 under [0054] to [0059], and then blended with at least one volatile non-solvent. When using, for example, two different cellulosic materials, such as cellulose triacetate and cellulose diacetate, it is preferable to disperse the cellulose-based materials separately from one another in a suitable volatile solvent, then to combine the two solutions thus formed and with at least one volatile non-solvent to set a desired final concentration of cellulose-based material based on the total casting solution.

A suitable concentration of cellulose-based material in the casting solution is in a range of about 3 to about 20% by weight, based on the total casting solution, with a concentration of 4 to 18% by weight being preferred. Most preferably, the concentration of cellulose-based material in the casting solution ranges from 5 to 15 percent by weight based on the total casting solution.

The volatile solvent is a solvent capable of dissolving a cellulose-based material, particularly a cellulose triacetate, a cellulose diacetate or a mixture of a cellulose triacetate and a cellulose diacetate, and has a boiling point in a range of about 20 ° C to about 65 ° C, preferably 25 ° C to 60 ° C, and most preferably 27 ° C to 35 ° C. Preferably, the volatile solvent is halogen-free. When using a halogen-free volatile solvent for the cellulose-based material, a particularly environmentally friendly process for producing the membrane of the invention can be provided.

Suitable volatile solvents (preferably halogen-free volatile solvents) having a boiling point in the range of about 20 ° C to about 65 ° C, preferably about 25 ° C to about 60 ° C are esters of aliphatic carboxylic acids having 2 to 5 carbon atoms, such as methyl acetate ( Boiling point: 57 ° C) and methyl formate (boiling point: 32 ° C), with methyl formate being particularly preferred. The volatile solvent is usually used in 30 to 80% by weight, preferably 40 to 75% by weight, and most preferably 50 to 70% by weight, based on the entire casting solution. In a particularly preferred embodiment, only methyl formate (within the scope of the above definition) is used as the volatile solvent for the cellulose-based material.

The volatile non-solvent is a liquid which is not suitable for completely dissolving a cellulose-based material such as cellulose triacetate, cellulose diacetate or a mixture thereof, and has a boiling point of not less than 60 ° C, preferably about 60 ° C about 150 ° C, more preferably about 70 ° C to about 110 ° C, on. The volatile non-solvent is preferably halogen-free.

Suitable volatile non-solvents are, for example, water, alcohols, such as methanol, ethanol and propanols, such as isopropanol, butanols, amyl alcohol, hexanols, heptanols and octanols, substituted or unsubstituted alkanes, such as hexane, heptane, octane and nitropropane, ketones, carboxylic acids, Ether and esters, with water (boiling point: 100 ° C), 2-propanol (boiling point: 82 ° C) and a mixture of water and 2-propanol are particularly preferred. The volatile non-solvent is usually in a range of 5 to 50 wt .-%, preferably in a range of 10 to 45 wt .-%, and most preferably in a range of 15 to 40 wt .-%, based on the entire casting solution, used.

The use of the specific combination of the volatile solvent and the volatile non-solvent is one of the essential factors for the formation of the specific microporous membrane according to the invention having two isotropic regions which surprisingly have the structure with the specific pore size distribution described above.

According to a particularly preferred embodiment of the present invention, the casting solution comprises a cellulosic material mixture of cellulose triacetate and cellulose diacetate, methyl formate as a volatile solvent and a mixture of 2-propanol and water as a volatile nonsolvent, optionally adding glycerol as an additive. Such a casting solution yielding a microporous membrane having excellent properties contains 3.3% by weight of cellulose triacetate, 3.3% by weight of cellulose diacetate, 63.271% by weight of methyl formate, 16.45% by weight of 2-propanol, and 13.678% by weight of water, and optionally up to 0.5% by weight of glycerol, based on the sum of all other components of the casting solution.

The casting or application of the casting solution in step (b) takes place in a customary manner, preferably on a flat base, such as a glass plate. The casting solution can be pulled out, for example, by means of a drawing carriage with a defined speed, wherein the desired thickness of the cast film can be adjusted as required. The thin deposited film described in step (b) is a wet film having a thickness in a range of about 500 μm to about 2500 μm, preferably about 800 μm to about 1500 μm, and most preferably about 1000 μm.

The predetermined temperature of the casting solution and / or the pad is üblicherwiese in a range of 10 ° C to 40 ° C, preferably 15 ° C to 30 ° C. The predetermined temperature of the casting solution and / or the pad is particularly preferably in a range of 12 to 20 ° C. The temperature of the casting solution and / or the base on which the casting solution is applied, it is set such that a controlled evaporation of the volatile solvent and the volatile non-solvent can be achieved, so that the above-described specific structure of the membrane of the invention can train.

In order to achieve the specific structure of the microporous membrane according to the invention, the controlled-composition atmosphere in step (c) of the method for producing a microporous membrane must be selected specifically. In the context of the present invention it has been found that a controlled composition atmosphere comprising at least the volatile solvent, the volatile non-solvent and the inert gas is essential for the formation of the specific structure of the membrane according to the invention. According to the invention, the controlled-composition atmosphere consists of the at least one volatile solvent, the at least one volatile non-solvent and the inert gas. In a preferred embodiment, the controlled-composition atmosphere comprises methyl formate, methyl acetate, 2-propanol, ethanol and water, the controlled-composition atmosphere particularly preferably containing methyl formate, water and 2-propanol. The temperature of the controlled composition atmosphere is usually maintained within a range of 10 ° C to 40 ° C, with a range of 18 ° C to 25 ° C being preferred to assist formation of the specific pore size distribution in the microporous membrane of the present invention.

The application of the specific evaporation step (c) in the process according to the invention under the controlled composition atmosphere described above is essential for the formation of the specific microporous membrane according to the invention having two discrete (isotropic) regions which surprisingly have a different average pore size.

In step (c) of the method for producing a microporous membrane, the term "until a white surface is formed on top of the mold solution film forming microporous membrane" for a required period of time at which the surface of the resulting microporous membrane turns white , This period may vary depending on the composition of the casting solution and the temperature of the casting solution and the temperature of the backing. In general, this period is under normal production conditions of 10 minutes to 35 minutes.

The drying of the microporous membrane described in step (d) is preferably carried out by passing an inert gas directly over the surface of the formed membrane. The inert gas used may be any customary inert gas, for example a noble gas such as argon or nitrogen. Nitrogen is preferred as an inert gas in the process of the present invention. The temperature of the inert gas stream used, preferably a nitrogen stream, is in the range of about 20 ° C to about 60 ° C, with a range of 25 ° C to 50 ° C being preferred for efficient removal of the volatile solvent and the volatile non-solvent Solvent to achieve.

The porous protective layers of the microporous membrane of the present invention arise as a result of the limited solubility of the cellulose-based material, for example, cellulose diacetate, in the non-solvent. Upon evaporation of the surface-enriched non-solvent forms the dissolved Share the protective layers. Accordingly, the protective layer of the upper side (gas side) at the end of the membrane formation arises shortly before the formation of the white surface, in that the proportion of the material dissolved in the non-solution phase partially covers the surface. The formation of the protective layer adjacent to the backing, which is less pronounced, is explained by the fact that the non-solvent does not cover the backing surface, but is present only in the pore channels before it exits the surface. Only a very small part of the dissolved material precipitates, thereby widening the pore walls.

The inventive microporous membrane can be prepared by the inventive method in a very environmentally friendly manner without using a halogenated solvent for the cellulose-based material and is particularly for the filtration of fluids in microfiltration, in particular for the pre-and final filtration of liquid media in the industry, in Laboratories and in the environmental field, excellently suited. Depending on the field of application, the membrane according to the invention can be produced both unreinforced and reinforced, for example nonwoven-reinforced.

1 shows a scanning electron micrograph of the controlled atmosphere exposed top (gas side) of a microporous membrane according to the invention (see Example 2 below) in 2000-fold magnification.

2 shows a scanning electron micrograph of the backing side of a microporous membrane according to the invention (see Example 2 below) in 2000-fold magnification.

3 shows a scanning electron micrograph of the membrane cross-section of a microporous membrane according to the invention (see Example 2 below) at 499-fold magnification, the top in the figure corresponds to the gas side and the bottom in the figure of the backing page.

4 shows the pore size distribution of the first isotropic region and the second isotropic region of a microporous membrane according to the invention (see Example 2 below).

The invention will be further elucidated by the following non-limiting examples.

Examples

Example 1 (Preparation of a casting solution)

In a first dispersing vessel, methyl formate is initially charged and cellulose triacetate (degree of acetylation: 58 to 62.5%) is added to a solids content of 10% by weight. After a dispersion time of 3 hours at 20 ° C to obtain a cloudy solution containing no visible gel particles. The cloudy solution is then cooled in a cooled to -55 ° C cooling bath in a cryostat over a period of about 90 minutes and stirred at low speed, so that the entire solution reaches a temperature of <-40 ° C. The solution temperature is measured via a PT-100 (thermocouple). Then the solution is taken out of the cooling bath. The solution is allowed to warm to room temperature while stirring and the solubility of the solution is checked with a transmission probe standardized to 100% transmission for pure methyl formate. The transmission should preferably be> 90%.

In a second dispersing vessel, after the introduction of methyl formate, cellulose diacetate (degree of acetylation: 51 to 57%) is added up to a solids content of 10% by weight. After a stirring time of 3 hours at 20 ° C to obtain a homogeneous solution.

The cellulose triacetate solution and the cellulose diacetate solution are proportionally mixed so that a total solids content of 10% by weight and a solid ratio of cellulose triacetate to cellulose diacetate of 1: 1 is achieved. Subsequently, a mixture of methyl formate, 2-propanol and water is added, so that a casting solution of the following composition was obtained: 3.3% by weight cellulose triacetate, 3.3% by weight cellulose diacetate, 63.271% by weight methyl formate, 16, 45% by weight of 2-propanol and 13.678% by weight of water. The casting solution was further added 0.5 wt .-% glycerol, based on the mass of the casting solution.

Example 2 (preparation of a membrane)

The casting solution prepared in Example 1 is drawn in a driven drawing slide at a defined speed in a chamber through which nitrogen flows on an encapsulated glass plate tempered to 18 ° C. The thickness of the casting solution film set with a pulling blade is 1000 μm. By Evaporation of the solvent and the non-solvent from the wet film produced, it forms a membrane structure. During membrane formation, the atmosphere is controlled above the forming membrane structure by defined flow guidance, preferably in the pulling direction at a distance of about 20 cm above the film. After completion of the membrane formation, recognizable by the now white surface of the membrane, the nitrogen stream is passed over the membrane surface over a period of 90 minutes in the drying phase to remove residual amounts of solvent and non-solvent. The membrane thus obtained shows the following physical measurement data, each as an average of 5 measurements (see Table 1). Table 1: Physical data of the membrane prepared in Example 2 Flow rate for water * [mL · min ^-1 · cm ^-2 · bar ^-1 ] Bubble point [bar] Bursting pressure [bar] Thickness [μm] 26.68 4.38 0.87 120.8 * Measured for 100 ml of water, 1 bar transmembrane pressure, 12.5 cm ² effective filter area

The flow is measured according to DIN 58355 (part 1). The measurement of the bubble pressure is in accordance with DIN 58355 (Part 2). The thickness of the unwetted membrane is measured by means of the thickness gauge No. 33105, Hahn and Kolb Werkzeuge GmbH, Stuttgart.

1 shows a scanning electron micrograph of the controlled atmosphere exposed top of the microporous membrane prepared in Example 2 at 2000-fold magnification.

2 shows a scanning electron micrograph of the glass plate side of the microporous membrane prepared in Example 2 in 2000-fold magnification.

3 shows a scanning electron micrograph of the membrane cross-section of the microporous membrane prepared in Example 2 at 499-fold magnification.

Out 3 showing the membrane cross-section of the microporous membrane prepared in Example 2, a significant asymmetry of the pore sizes between the upper surface exposed to the controlled composition and the glass plate side of the membrane can be seen. In a visual evaluation of the maximum pore sizes of the top and the glass plate side, the following parameters result: Pore diameter of the top (P _O ): about 6.0 μm Pore diameter of the glass plate side (P _G ): about 1.60 microns Ratio P _O / P _G = 3.75

The specific structure comprising two isotropic regions of the microporous membrane prepared in Example 2 can be further characterized by determining the absolute mass flow rate of a defined sugar test solution (10% by weight raw sugar (from sugarcane) in water, 20 ° C). For a flow of the top or the glass plate side of the microporous membrane of Example 2, a ratio of 3.4: 1 for the maximum filterable to the membrane block mass of the sugar test solution is determined.

Porometric measurements (according to US standard ASTM 1294-89, performed with an APP-1200 AEXI capillary flowometer from Porous Materials Inc. (PMI), New York) provide the following parameters (measured assuming cylindrical pores with a PMI porometer): Largest membrane pore: 0.426 μm Smallest membrane pore: 0.152 μm Ratio largest membrane pore / smallest membrane pore: 2.80 Most common membrane pore: 0.395 μm

The wetted membrane is subjected to increasing pressure (air or nitrogen) and the gas volume flow is measured through the membrane as a function of pressure. These volume flows (wet curve) are set in relation to the volume flows which are measured as a function of the pressure through the unwetted membrane (dry curve). Thus, pore size and number can be measured.

The average pore size ratio P _O / P _G for the microporous membrane prepared in Example 2 was determined to be 3.3: 1.

4 shows the normalized to the most common membrane pore pore size distribution of the isotropic regions of the microporous membrane prepared in Example 2. Out 4 shows that the pore size distribution in the two isotropic regions is very narrow. In the first isotropic region, the plurality of pores has a pore diameter of about 0.4 μm, and in the second isotropic region, the plurality of pores has a pore diameter of about 0.1 μm. The isotropic regions of the microporous membrane are characterized by a substantially uniform pore size. The number of pores in the transition between the two isotropic regions is relatively small, which can be seen from the low normalized frequencies for pore sizes in the range from 0.225 μm to 0.275 μm.

Claims (18)

A process for producing a cellulose-based microporous membrane having a porous protective layer as a first membrane surface and a porous protective layer as a second membrane surface and a porous sponge-like structure between these two protective layers, the sponge-like structure consisting of two isotropic regions, and the pores of the first isotropic region, which is adjacent to the first membrane surface, smaller than the pores in the protective layer, which is the first membrane surface, but larger than the pores of the second isotropic region, and the pores of the second isotropic region are smaller than the pores in the protective layer, which represents second membrane surface, the method comprising the steps: (a) preparing a homogeneous casting solution comprising a cellulosic material containing at least one volatile solvent for the cellulosic material and at least one volatile non-solvent for the cellulosic material, (b) pouring the casting solution onto a base to form a thin film under inert gas at a predetermined temperature; (c) evaporating the volatile solvent and the volatile non-solvent under a controlled composition atmosphere consisting of the at least one volatile solvent, the at least one volatile non-solvent and the inert gas, to form a white surface on top of the from the casting solution film forming microporous membrane and (D) drying the microporous membrane formed by overflowing the membrane surface with the inert gas. The method of claim 1, wherein the volatile solvent is halogen-free. A process according to claim 1 or 2, wherein the volatile solvent is an ester of an aliphatic carboxylic acid having 2 to 5 carbon atoms. Method according to one or more of claims 1 to 3, wherein the volatile solvent is methyl formate and / or methyl acetate. Method according to one or more of claims 1 to 4, wherein a reinforcing material is integrated into the membrane. A cellulosic microporous membrane obtained by a method according to any one of claims 1 to 5, wherein the microporous membrane has a porous protective layer as a first membrane surface and a porous protective layer as a second membrane surface and a porous sponge-like structure between these two protective layers, the sponge-like structure comprising two isotropic regions, and the pores of the first isotropic region adjacent to the first membrane surface are smaller than the pores in the protective layer which is the first membrane surface but larger than the pores of the second isotropic region and the pores of the second one isotropic region smaller than the pores in the protective layer, which represents the second membrane surface. The microporous membrane of claim 6, wherein the pore density in the porous protective layers is in a range of 1 × 10 ⁴ to 1 × 10 ⁶ pores / mm ² . A microporous membrane according to claim 6 or 7, wherein the pore density in the porous protective layer constituting the first membrane surface is in a range of 1 x 10 ⁴ to 5 x 10 ⁵ pores / mm ² . A microporous membrane according to one or more of claims 6 to 8, wherein the pore density in the porous protective layer constituting the second membrane surface is in a range of 3 x 10 ⁵ to 1 x 10 ⁶ pores / mm ² . A microporous membrane according to one or more of claims 6 to 9, wherein the pores in the porous protective layer constituting the first membrane surface have an average diameter in a range of 0.4 μm to 15 μm. The microporous membrane according to one or more of claims 6 to 10, wherein the pores in the porous protective layer constituting the second membrane surface have an average diameter in a range of 0.1 μm to 3.0 μm. Microporous membrane according to one or more of claims 6 to 11, wherein the pores in the isotropic regions, based on the average pore diameter, differ by a factor of 1.4 to 16. A microporous membrane according to one or more of claims 6 to 12, wherein one of the isotropic regions constitutes 20% to 80% of the thickness of the porous sponge-like structure. Microporous membrane according to one or more of claims 6 to 13, wherein the cellulose-based material comprises at least one cellulose ester. The microporous membrane of claim 14, wherein the cellulose ester is at least one cellulose acetate. The microporous membrane of claim 15, wherein the cellulose acetate comprises a cellulose triacetate, a cellulose diacetate or a mixture thereof. Microporous membrane according to one or more of claims 6 to 16, wherein the cellulose-based material comprises a mixture of cellulose triacetate having a degree of acetylation in a range of 58% to 62.5% and cellulose diacetate having a degree of acetylation in a range of 51% to 57% in a weight ratio of 1.5: 1 to 0.8: 1. Microporous membrane according to one or more of claims 6 to 17, wherein a reinforcing material is integrated into the membrane.

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